21.(25520-!-item-!-188;#058&002357)
Before scientists learned how to make a synthetic growth hormone, removing it painstakingly in small amounts from the pituitary glands of human cadavers.
(A) scientists learned how to make a synthetic growth hormone, removing it painstakingly
(B) scientists had learned about making a synthetic growth hormone, they had to remove it painstakingly
(C) scientists learned how to synthesize the growth hormone, it had to be painstakingly removed(答案)
(D) learning how to make a synthetic growth hormone, scientists had to remove it painstakingly
(E) learning how to synthesize the growth hormone, it had to be painstakingly removed by scientists
D 选项 为什么不好呀 是不是 before doing 本身就是不好呀 牛牛们 帮我解释一下 谢谢
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发表于 2011-3-22 20:20:53
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